Ugly, alike.
This is exported PNG in original size 256x256 px (100% size)
Rasterized rectangle (with horizontal gradient fill) from Affinity Designer:
My software:
Correct picture must be composited from vertical lines. Or not?
(For testing u can use FloodFill with 0% tolerance, or Designer with "Pixel view mode")
I working mostly in 16bit RGB. Quality result is important for me.